Despite improvements in nutritional management of low birth weight (LBW) infants over the past decade, most who weigh <1500g at birth, even if appropriate size for gestational age at birth, weigh less than and are shorter than the 10th percentiles of intrauterine standards at discharge (approximately 36 wks post menstrual age) and many remain small as late as adolescence and young adulthood. While the consequences of the early growth failure are not known with certainty, there is evidence that it may contribute to the high prevalence of developmental deficits in this population. Studies are proposed to test the hypothesis that there is a finite period during which the human infant is capable of maximal utilization of protein for growth. Based on limited studies in infants and acute studies in animals, this period appears to encompass the latter part of gestation and the first few months post-term but it remains undefined. Corollary hypotheses, which also will be tested in human infants, are that maximizing protein intake during this finite period of enhanced sensitivity to protein intake will improve growth and, hence, reduce the subsequent growth and neurodevelopmental deficits of preterm infants. In toto, the proposed studies will determine if there is a finite period during which the human neonate can maximally utilize protein intake for growth as measured by whole body protein turnover and also will define this period. They also will determine if maximizing protein intake during this finite period improves early growth without imposing unacceptable metabolic consequences and if this improved growth reduces subsequent growth and neurodevelopmental deficits. Overall, the findings of this project are expected to contribute to further improvements in nutritional management and, hence, outcome of LBW infants.